It is well known to package small items and finer bulk materials in sealed thermo-plastic bags. Such packages are known as particularly effective in protecting the products sealed therein and in facilitating viewing of the contents without opening the bag.
Systems, apparatus and methods have been devised to effect automatic and semi-automatic packaging in such bags. Examples of such apparatus, systems and methods are shown in U.S. Pat. Nos. 3,948,015, issued on Apr. 6, 1976 and in 3,965,653, issued on June 29, 1976, both of which are herein expressly incorporated by reference.
Commercial equipment has been sold for performing these automatic and semi-automatic packaging functions, two exemplary machines being known as the Model H-100 and the Model H-55, manufactured and sold by Automated Packaging Systems, Inc. of Twinsburg, Ohio, USA.
Packaging apparatus and systems embodied by the above identified machines use webs of interconnected heat-sealable thermo-plastic bags. The bags are successively fed to a loading and sealing station where they are opened by an air flow system and loaded.
The loaded bags are successively sealed and then detached one from another.
In sealing, a member grips each loaded bag successively and isolates a region of the bag where a heat seal is to be formed. A heater element is moved, relative to the isolated region, proximate or impinging against the desired seal area. Electric power applied to the heater element heats the seal area region to a temperature sufficiently high to effect a heat seal of the loaded bag. Subsequently, the loaded bags are successively detached from the web. Alternately, the heater element is stationary, and a pressure member forces the seal area against the heater.
The apparatus described above is usually employed to make a long rapid succession of seals of successive bags. In such an application, the heater element is kept hot, i.e., maintained at approximately a uniform temperature.
Other types of sealing operations, however, are also of interest. Continuous sealing is a type of sealing in which a continuous heated heater element is maintained in substantially continuous contact with moving material to be sealed. Such continuous sealing can be applied, for example, in making of the heat sealable plastic bags. To do this, one begins with a length of large diameter plastic tubing. The tubing is laid flat, and a "hot knife" heater element is moved relatively along the longitudinal direction of the large tube, which makes a longitudinal seal extending along the large tube, in order to convert the large diameter tube to two or more smaller diameter tubes.
Still another sealing application is of the intermittent variety, such as where low volume bag sealing is done at time intervals which may be spaced a varying and considerable amount, one from another. In such applications, temperature control is very important, since intermittent application results in thermal transients occurring within the machine and the heater element which can interfere with good sealing. If the intervals are short, thermal build-up can occur and the heater element can become too hot. On the other hand, if intervals between sealing cycles are long, insufficient heat build-up may take place, and the heater element will be too cold for effective sealing.
Maintenance of heating element temperature to within a relatively precise range is important for making consistently good quality heat seals. If the heater element temperature is too low, the material to be sealed will not be sufficiently softened and sealing may be incomplete. If, on the other hand, heater element temperature is too high, the thermo-plastic material to be sealed will melt excessively, which can result in burning through one or both layers of plastic material and will cause the seal to be defective.
According to one proposal for temperature control, line voltage is coupled to the heating element and its circuit element values are simply selected to maintain the heating element at a predetermined desired temperature as long as the line voltage is applied. This proposal suffers from the disadvantage that, if line voltage changes, so will heating element temperature, and sometimes this change can be sufficient to adversely affect the quality of the seals made.
According to another proposal, the temperature of the heating element is monitored by a heat sensing device, such as a thermocouple or the like, and output from the thermocouple is sensed. This output is connected in feedback fashion to control circuitry for adjusting the energy applied to the heating element. A problem with this approach is that the accuracy of such temperature monitoring devices is sometimes not sufficiently great. A further problem results from the fact that such temperature monitoring devices monitor the temperature of the heater element from a distance which, though small, can result in inaccurate temperature indications. This is particularly true where ambient temperature is relatively low, or where drafts are present.
Another proposal has been to use a thermistor positioned and coupled to cut off application of power to the heating element for a significant period of time when the heating element is removed from proximity to the seal area.
Another proposal has been to employ a temperature sensitive element near the heater element, whose resistance changes with temperature, and to utilize that change in resistance to actuate control circuitry for adjusting power applied to the heater element.
These last two proposals suffer from the same disadvantage as the previously mentioned proposal, in that temperature sensing is done by sensing heat output from the heater element, rather than by directly sensing the temperature of the heater element itself.
Thus far, this discussion has been primarily directed to the type of repetitive sealing operation wherein a heating element is desired to be held at a substantially uniform heat sealing temperature and the element is rapidly applied to a succession of desired seal areas.
Another type of desirable seal is a seal having a relatively large seal area on which indicia is embossed. Such indicia can comprise for example, the logo or name of the entity which manufactures or sells the bag contents. Another use can comprise indication of the identity of the person or opertor who performed the sealing operation, or who counted or inspected the contents of the bag.
A practical application of such a use is the packaging of precounted amounts of currency in plastic bags for banks.
Embossed seals also have utility for security applications, where it is desirable to provide a seal such that the container cannot easily be opened and resealed by an unauthorized person without the fact of such intrusion being detectable. For example, one cannot, without leaving evidence, open a container known to have an embossed seal and simply reseal it with a plain heater element having no indicia.
Even if it is not desired to emboss into the seal area a logo, name or number, it may nonetheless be desirable for security purposes to emboss a predetermined geometrical pattern into the seal area which is relatively difficult to duplicate.
The procedure for making embossed seals is somewhat different from that described above. To make an embossed seal, the heater element is made in the form of a bar, mating with a clamping surface bearing raised pattern corresponding to the desired indicia. The layers of the container to be sealed are then inserted between the jaws of a clamp apparatus, at least one portion of which bears or consists of the heating element. The clamp is closed prior to application of electric power to the heater element. Electric power is then applied to the heater element which heats the area desired for the seal to a point at which the seal is effected. Power is then cut off from the heating element and the clamped assembly is allowed to cool, after which the clamp is released and the sealed container removed prior to the initiation of another sealing cycle.
Private tests have shown that good quality embossed seals are difficult to make. Legibility of the indicia has sometimes appeared inconsistent with making seals with good mechanical properties. Seal quality control has been sought by adjusting the time of down application to the resistive heating element. One problem with this approach appears to arise in instances in which a long succession of sealing cycles is needed. Presumably because of transient thermal conditions during a succession of sealing operations, the duration of heat application to effect a good seal varies considerably over a sequence of seals. Such time durations have been seen to vary from between 11/2 seconds to about 0.3 seconds. This need for continuous timing control has made impractical the making of successions of embossed seals.
The necessity for varying the time of heat application has been a substantial obstacle to making rapid successions of good embossed heat seals.
It is a general object of the present invention to provide a heat sealing method and circuit for effecting, as desired, (1) precise maintenance of a heat sealing element at substantially a predetermined temperature and (2) operation of a heat sealing element in a uniformly timed series cycles for making consistently good embossed seals.